Jackfruit: Composition, structure, and progressive collapsibility in the largest fruit on the Earth for impact resistance.

The jackfruit is the largest fruit on the Earth, reaching upwards of 35 kg and falling from heights of 25 m. To survive such high energy impacts, it has evolved a unique layered configuration with a thorny exterior and porous tubular underlayer. During compression, these layers exhibit a progressive collapse mechanism where the tubules are first to deform, followed by the thorny exterior, and finally the mesocarp layer in between. The thorns are composed of lignified bundles which run longitudinally from the base of the thorn to the tip and are embedded in softer parenchymal cells, forming a fiber reinforced composite. The mesocarp contains more lignin than any of the other layers while the core appears to contain more pectin giving rise to variations in compressive and viscoelastic properties between the layers. The surface thorns provide a compelling impact-resistant feature for bioinspiration, with a cellular structure that can withstand large deformation without failing and wavy surface features which densify during compression without fracturing. Even the conical shape of the thorns is valuable, presenting a gradually increasing surface area during axial collapse. A simplified model of this mechanism is put forward to describe the force response of these features. The thorns also distribute damage laterally during impact and deflect cracks along their interstitial valleys. These phenomena were observed in 3D printed, jackfruit-inspired designs which performed markedly better than control prints with the same mass. STATEMENT OF SIGNIFICANCE: Many biological materials have evolved remarkable structures that enhance their mechanical performance and serve as sources of inspiration for engineers. Plants are often overlooked in this regard yet certain botanical components, like nuts and fruit, have shown incredible potential as blueprints for improved impact resistant designs. The jackfruit is the largest fruit on Earth and generates significant falling impact energies. Here, we explore the jackfruit's structure and its mechanical capabilities for the first time. The progressive failure imparted by its multilayered design and the unique collapse mode of the surface thorns are identified as key mechanisms for improving the fruit's impact resistance. 3D printing is used to show that these structure-property benefits can be successfully transferred to engineering materials.

Histones and their chaperones: Adaptive remodelers of an ever-changing chromatinic landscape.

Chromatin maintenance and remodeling are processes that take place alongside DNA repair, replication, or transcription to ensure the survival and adaptability of a cell. The environment and the needs of the cell dictate how chromatin is remodeled; particularly where and which histones are deposited, thus changing the canonical histone array to regulate chromatin structure and gene expression. Chromatin is highly dynamic, and histone variants and their chaperones play a crucial role in maintaining the epigenetic regulation at different genomic regions. Despite the large number of histone variants reported to date, studies on their roles in physiological processes and pathologies are emerging but continue to be scarce. Here, we present recent advances in the research on histone variants and their chaperones, with a focus on their importance in molecular mechanisms such as replication, transcription, and DNA damage repair. Additionally, we discuss the emerging role they have in transposable element regulation, aging, and chromatin remodeling syndromes. Finally, we describe currently used methods and their limitations in the study of these proteins and highlight the importance of improving the experimental approaches to further understand this epigenetic machinery.

Equine hoof wall: Structure, properties, and bioinspired designs.

The horse hoof wall exhibits exceptional impact resistance and fracture control due to its unique hierarchical structure which contains tubular, lamellar, and gradient configurations. In this study, structural characterization of the hoof wall was performed revealing features previously unknown. Prominent among them are tubule bridges, which are imaged and quantified. The hydration-dependent viscoelasticity of the hoof wall is described by a simplified Maxwell-Weichert model with two characteristic relaxation times corresponding to nanoscale and mesoscale features. Creep and relaxation tests reveal that the specific hydration gradient in the hoof keratin likely leads to reduced internal stresses that arise from spatial stiffness variations. To better understand realistic impact modes for the hoof wall in-vivo, drop tower tests were executed on hoof wall samples. Fractography revealed that the hoof wall's reinforced tubular structure dominates at lower impact energies, while the intertubular lamellae are dominant at higher impact energies. Broken fibers were observed on the surface of the tubules after failure, suggesting that the physically intertwined nature of the tubule reinforcement and intertubular matrix improves the toughness of this natural fiber reinforced composite. The augmented understanding of the structure-mechanical property relationship in dynamic loading led to the design of additively manufactured bioinspired structures, which were evaluated in quasistatic and dynamic loadings. The inclusion of gradient structures and lamellae significantly reduced the damage sustained in drop tower tests, while tubules increased the energy absorption of samples tested in compact tension. The samples most similar to the hoof wall displayed remarkably consistent fracture control properties. STATEMENT OF SIGNIFICANCE: The horse hoof wall, capable of withstanding large, repeated, dynamic loads, has been touted as a candidate for impact-resistant bioinspiration. However, our understanding of this biological material and its translation into engineered designs is incomplete. In this work, new features of the horse hoof wall are quantified and the hierarchical failure mechanisms of this remarkable material under near-natural loading conditions are uncovered. A model of the hoof wall's viscoelastic response, based on studies of other keratinous materials, was developed. The role of hydration, strain rate, and impact energy on the material's response were elucidated. Finally, multi-material 3D printed designs based on the hoof's meso/microstructure were fabricated and exhibited advantageous energy absorption and fracture control relative to control samples.